The present invention is directed to bicycle transmissions and, more particularly, to an internally mounted multi-speed hub transmission for a bicycle.
An internally-mounted multi-speed hub transmission sometimes is mounted to the rear wheel of a bicycle so that the rider can select different gear ratios to vary the pedaling effort. A typical hub transmission includes a hub axle that is mounted to the bicycle frame, a driver rotatably supported to the hub axle for receiving the pedaling force through a sprocket and chain, and a hub shell rotatably supported to the hub axle. A power transmitting mechanism is disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths, wherein each power transmission path typically produces a unique gear ratio. The power transmitting mechanism ordinarily comprises a planetary gear mechanism including one or more sun gears rotatably supported around the hub axle, a ring gear rotatably supported around the hub axle, a planet gear carrier rotatably supported around the hub axle, and a plurality of planet gears rotatably supported to the planet gear carrier and meshing with the sun gear and the ring gear. The plurality of power transmission paths and the corresponding gear ratios are selected by selectively nonrotatably coupling the various components to each other. For example, one gear ratio may be selected by nonrotatably coupling a sun gear to the hub axle, another gear ratio may be selected by nonrotatably coupling the driver relative to the planet gear carrier, and another gear ratio may be selected by nonrotatably coupling the driver relative to the ring gear. Many such coupling relationships often are possible in a typical hub transmission, thus resulting in a relatively large number of possible gear ratios.
A shift mechanism is usually provided for selecting the plurality of power transmission paths. The shift mechanism may comprise a shift sleeve that surrounds the axle such that rotation of the shift sleeve controls the nonrotatable coupling of the various components. Such a shift sleeve ordinarily is coupled to an actuator member outside the hub, wherein rotation of the actuator member is controlled by a shift control device mounted to the handlebar or by a motor that is electronically controlled by the rider. When the rider exerts a large amount of force on the pedals to accelerate the bicycle quickly, a very large amount of force is applied to the internal components of the hub, thus creating significant resistance to the shifting operation. Such resistance results in excessive manual shifting effort required by the rider or in unacceptable strain on the motor that drives the actuator member.
U.S. Pat. No. 6,572,508 discloses an apparatus that uses the rotational power of the hub itself to assist the shifting operation when significant drive force is applied to the hub. That apparatus senses when the shift sleeve experiences significant resistance to the shifting operation. Such resistance activates a pawl mechanism coupled to the shift sleeve so that pawls carried by the pawl mechanism engage a ratchet mechanism comprising a plurality of ratchet teeth formed on the inner peripheral surface of the driver. The rotational power of the driver is thus communicated to the shift sleeve, and the shift sleeve is rotated to complete the shifting operation. If the shift sleeve still does not move in response to the rotational power of the driver, then the pawls ride over the originally engaged ratchet teeth and engage successive ratchet teeth in the rotational direction, thereby providing a series of pulses of rotational power to the shift assist mechanism.
U.S. Pat. No. 6,641,500 also discloses an apparatus that uses the rotational power of the hub itself to assist the shifting operation when significant drive force is applied to the hub. To avoid damage to the shift sleeve or to other components of the shift mechanism, a power control mechanism that comprises at least two frictional discs controls an amount of rotational power communicated from the driver to the shift mechanism to prevent locking between the driver and the shift mechanism.